The present invention resides in a process for manufacturing lacquered rings of aluminum or tin-plate for use as lids on food cans wherein the inner cut edge is turned over outwards.
Today, cans represent favorably priced containers which can withstand knocks and heavy handling. In addition, cans can be used to hold a large variety of foodstuffs. Various opening systems have been developed so as to enable the customer to open the cans without the need of any special tools.
For example, there are lids made of aluminum or tin-plate which can be torn open along a line of weakness by means of a flap or a ring secured to a hollow rivet shaped out of the lid. Such lids, which can also withstand sterilization treatment, lead to the exposure of sharp, cut edges which represent a considerable hazard to the user. Furthermore, if the indentation forming the line of weakness is insufficient, the opening of the can is possible only by applying excessive force. On the other hand, if the indentation is too deep, then there is a danger of the lid being penetrated accidentally. A lid with a line of weakness is more sensitive to knocks which could cause the can to burst open. Another disadvantage which must be taken into account is that failure of the material can occur due to corrosion at the line of weakness caused by an aggressive content. Such corrosive attack is known to occur preferentially in those regions where the material has been heavily deformed.
There are other known can lids which comprise a ring having a tear-back membrane made of aluminum sealed to it. To reduce the risk of injury, the sharp edge on the opening in the ring, which results from the manufacture of the ring, is bent over inwards. Such lids have proved useful for cans containing dry substances. However, when the contents contain water and in particular when the contents is aggressive, these lids are not suitable, as the cut edge which is turned inwards is in contact with the contents. Consequently, in particular during the sterilization process, the cut edge is exposed to corrosive attack which causes contamination of the contents and reduces their value. Today it is not technically possible to subsequently apply a perfect lacquer coating to the cut edge. Furthermore, for economic reasons, it would not be justifiable. In addition, lacquering the cut edge would not reduce the risk of injury to the user on removing the contents from the can.
In addition to the foregoing, there are light weight containers which are corrosion resistant, able to withstand sterilization and easy to open. These containers are made of aluminum coated in plastic and are closed via a sealed seam. The main disadvantage of these containers is their lack of rigidity.